While methotrexate has been in clinical use as an antitumor agent for a number of years, the recent introduction of high-dose schedules followed by citrovorum factor rescue in the treatment of solid tumors has resulted in renewed interest in this agent, and has revealed also that our knowledge of the clinical pharmacology of this and related folate antagonists is still incomplete. The aim of this project is to carry out studies on the pharmacokinetics of high-dose methotrexate; on the biological role in human subjects and in experimental animals of the recently-discovered diglutamate metabolite of the drug (4-amino-4-deoxy- N10-methylpteroylglutamyl-gamma-glutamate); and on the pharmacology of intrathecally administered methotrexate.